Disk drives are commonly used in workstations, personal computers, laptops and other computer systems to store large amounts of data in a form that can be made readily available to a user. In general, a disk drive comprises a magnetic disk that is rotated by a spindle motor. The surface of the disk is divided into a series of data tracks. The data tracks are spaced radially from one another across a band having an inner diameter and an outer diameter.
Each of the data tracks extends generally circumferentially around the disk and can store data in the form of magnetic transitions within the radial extent of the track on the disk surface. An interactive element, such as a magnetic transducer, is used to sense the magnetic transitions to read data, or to transmit an electric signal that causes a magnetic transition on the disk surface, to write data. The magnetic transducer includes a reader and a writer that contain the active elements of the transducer at a position suitable for interaction with the magnetic surface of the disk. The radial dimension of the reader and the writer elements in the interactive element, fits within the radial extent of the data track containing the transitions, so that only transitions of the single track are transduced by the interactive element when the interactive element is properly centered over the respective data track.
The magnetic transducer is mounted by a head structure to a rotary actuator arm and is selectively positioned by the actuator arm over a preselected data track of the disk to either read data from or write data to the preselected data track of the disk, as the disk rotates below the transducer. The actuator arm is, in turn, mounted to a voice coil motor that can be controlled to move the actuator arm across the disk surface.
A servo system is typically used to control the position of the actuator arm to insure that the head is properly centered over the magnetic transitions during either a read or write operation. In the servo system, servo position information is recorded on the disk surface between written data blocks, and periodically read by the head for use in a closed loop control of the voice coil motor to position the actuator arm. Such a servo arrangement is referred to as an embedded servo system.
In modern disk drive architectures utilizing an embedded servo, each data track is divided into a number of data sectors for storing fixed sized data blocks, one per sector. Associated with the data sectors are a series of servo sectors, generally equally spaced around the circumference of the data track. The servo sectors can be arranged between data sectors or arranged independently of the data sectors such that the servo sectors split data fields of the data sectors.
Each servo sector contains magnetic transitions that are arranged relative to a track centerline such that signals derived from the transitions can be used to determine head position. For example, the servo information can comprise two separate bursts of magnetic transitions, one recorded on one side of the track centerline and the other recorded on the opposite side of the track centerline. Whenever a head is over a servo sector, the head reads each of the servo bursts and the sensed signals are transmitted to, e.g., a microprocessor within the disk drive for processing.
When the head is properly positioned over a track centerline, the head will straddle the two bursts, and the strength of the combined signals transduced from the burst on one side of the track centerline will equal the strength of the combined signals transduced from the burst on the other side of the track centerline. The microprocessor can be used to subtract one burst value from the other each time a servo sector is read by the head. When the result is zero, the microprocessor will know that the two signals are equal, indicating that the head is properly positioned.
If the result is other than zero, then one signal is stronger than the other, indicating that the head is displaced from the track centerline and overlying one of the bursts more than the other. The magnitude and sign of the subtraction result can be used by the microprocessor to determine the direction and distance the head is displaced from the track centerline, and generate a control signal to move the actuator back towards the centerline.
A closed-loop track-following servo, as described above, is sufficient for track following in the absence of major irregularities of the recording medium or drive mechanics. An entirely closed-loop system has a harder time correcting for larger and more abrupt irregularities. For certain kinds of disc drive irregularities, however, a combination of feedback and feedforward techniques can be employed to enhance the track-following ability of the servo control system.
Repeatable Run Out (RRO) is an actuator arm positioning error that occurs regularly with each revolution of the disc. A typical situation in which RRO occurs is when the tracks on the disc are not perfect circles, although one of ordinary skill in the art will recognize that this is not the only situation in which RRO occurs. In such cases, the track-following servo must cause the actuator arm to move radially with respect to the disc to roughly the same degree at each revolution. It is a well-known practice in the art to measure the degree of RRO (i.e., the amount of actuator arm movement necessary to compensate for RRO) and store this information on the disc or in a non-volatile memory, so that a feedforward signal may be applied to the servo loop in order to proactively compensate for RRO. This measurement of the degree of RRO is called “RRO estimation.” An example of using stored parameters from RRO estimation to compensate for RRO is provided by commonly assigned U.S. Pat. No. 5,585,976 to Ich V. Pham.
“RRO estimation” is aptly named, since an exact measurement is virtually impossible, from a practical standpoint. That is because the actual positioning errors experienced by the actuator arm and the actual correction that must be applied to those errors are dependent on more than just RRO. RRO is a steady-state error. Disc drives, exhibit both steady-state and transient behavior. The transient behavior of a disc drive actuator arm is dependent on many factors, including external forces (e.g., operating vibration and shock on a disk drive) or the physical characteristics of the drive itself (e.g., a resonant frequency of the drive).
Since disc drives exhibit both steady-state errors (RRO) and transient errors (non-repeatable runout or NRRO), a goal of RRO estimation is to reduce or eliminate the effects of NRRO in the measurements taken. An existing strategy for addressing this problem is to take a series of measurements from a number of consecutive revolutions of the disc and use averaging or some other mathematical technique to reduce the “outliers” in the measurement data so as to achieve a realistic estimate of the RRO. This process is typically done as part of the manufacturer's media certification process, in which the disc drive is scanned for defects in the recording medium.
While this existing approach is theoretically sound, this approach is not immune from problems. In particular, sometimes a positioning error that resembles RRO will actually be due to NRRO. For example, an external force applied to the disc drive may cause the actuator arm or recording medium to move in a periodic fashion, due to mechanical resonances in the arm's natural response characteristic. Over a number of consecutive revolutions of the disc, this periodic behavior may not be readily distinguishable from RRO, and the RRO compensation estimated during that period may contain spurious compensation data as a result.
Thus, a need exists for an RRO estimation scheme that efficiently minimizes the introduction of spurious compensation data due to NRRO. The present invention provides a solution to this and other problems, and offers other advantages over previous solutions.